Which is a continuation-in-part of U.S. patent application Ser. No. 07/320,420 filed Mar. 8, 1989 now U.S. Pat. No. 4,956,127 issued Sep. 11, 1990.
1. Field of the Invention
The present invention pertains to gas-liquid contacting trays and, more particularly, an improved valve-tray assembly incorporating directional thrust valves and tray construction for higher efficiency operation.
2. History of the Prior Art
Distillation columns are utilized to separate selected components from a multicomponent stream. Generally, such gas-liquid contact columns utilize either trays, packing or combinations thereof. In recent years the trend has been to replace the so-called "bubble caps" by sieve and valve trays in most tray column designs, and the popularity of packed columns, either random (dumped) or structured packing have been utilized in combination with the trays in order to effect improved separation of the components in the stream.
Successful fractionation in the column is dependent upon intimate contact between liquid and vapor phases. Some vapor and liquid contact devices, such as trays, are characterized by relatively high pressure drop and relatively high liquid hold-up. Another type of vapor and liquid contact apparatus, namely structured high efficiency packing, has also become popular for certain applications. Such packing is energy efficient because it has low pressure drop and low liquid hold-up. However, these very properties at times make columns equipped with structured packing difficult to operate in a stable, consistent manner. Moreover, many applications simply require the use of trays.
Fractionation column trays come in two configurations: cross-flow and counter flow. The trays generally consist of a solid tray or deck having a plurality of apertures and are installed on support rings within the tower. In cross-flow trays, vapor ascends through the apertures and contacts the liquid moving across the tray, through the "active" area thereof. In this area, liquid and vapor mix and fractionation occurs. The liquid is directed onto the tray by means of a vertical channel from the tray above. This channel is referred to as the Inlet Downcomer. The liquid moves across the tray and exits through a similar channel referred to as the Exit Downcomer. The location of the downcomers determines the flow pattern of the liquid. If there are two Inlet Downcomers and the liquid is split into two streams over each tray, it is called a two pass tray. If there is only one Inlet and one Outlet Downcomer on opposite sides of the tray, it is called a single pass tray. For two or more passes, the tray is often referred to as a Multipass Tray. The number of passes generally increases as the required (design) liquid rate increases. It is the active area of the tray, however, which is of critical concern.
Not all areas of a tray are active for vapor-liquid contact. For example, the area under the Inlet Downcomer is generally a solid region. To attempt to gain more area of the tray for vapor/light contact, the downcomers are often sloped. The maximum vapor/liquid handling capacity of the tray generally increases with an increase in the active or Bubbling Area. There is, however, a limit as to how far one can slope the downcomer(s) in order to increase the Bubbling Area otherwise the channel will become too small. This can restrict the flow of the liquid and/or restrict the disengagement of vapor retained in the liquid, cause liquid to back up in the downcomer, and thus prematurely limit the normal maximum vapor/liquid handling capacity of the tray. The present invention specifically addresses the problem of restricted disengagement of vapor retained in the liquid.
A variation for increasing the Bubbling Area and hence vapor/liquid handling capacity is a Multiple Downcomer (MD) tray. There is usually a plurality of box shaped vertical channels installed in a symmetrical pattern across the tray to direct liquid onto and off of the tray. The downcomers do not extend all the way to the tray below but stop short of the tray by a predetermined distance which is limited by a sufficient space to permit disengagement of any vapor retained in the liquid entering the Exit Downcomer. The downcomer pattern is rotated 90 degrees between successive trays. The bottom of the boxes is solid except for slots that direct the liquid onto the Bubbling Area of the tray below, in between the outlet downcomers of the tray. The MD tray falls into the category of Multipass Trays and is usually used for high liquid rates.
Addressing now select cross flow plate designs, a particularly effective tray in process columns is the sieve tray. This tray is constructed with a large number of apertures formed in the bottom surface. The apertures permit the ascending vapor to flow into direct engagement with the liquid that is flowing across the tray from the downcomer described above. When there is sufficient vapor flow upwardly through the tray, the liquid is prevented from running downwardly through the apertures (referred to as "weeping"). A small degree of weeping is normal in trays while a larger degree of weeping is detrimental to the capacity and efficiency of a tray.
Tray efficiency is also known to be improved in sieve type trays by increasing the froth height of the liquid and reducing the backflow of the liquid flowing across the tray. Froth is created when vapor bubbles percolate upwardly through the liquid flowing across the tray. The suspension of the vapor in the liquid prolongs the vapor liquid contact which enhances the efficiency of the process. The longer the froth is maintained and the higher the froth is established, the greater the vapor liquid retention. Higher froth requires smaller vapor bubbles and the formation of the bubbles at a sufficiently slow rate. Likewise, backflow occurs beneath the froth when circulating currents of liquid are established during the liquid flow across the plate. This generally forms along the lateral portions thereof. These currents carry liquid back across the tray in a manner that reduces the concentration-difference driving force for mass transfer. It is the concentration-difference between the vapor and the liquid which enhances the effectiveness of the vapor-liquid contact.
The concentration-difference between the vapor and the liquid can be effected in many ways; some reducing efficiency. For example, as operating pressure increases, descending liquid begins to absorb vapor as it moves across a tray. This is above that normally associated as dissolved gas as governed by Henry's Law and represents much larger amounts of vapor bubbles that are commingled or "entrained" with the liquid. This vapor is not firmly held and is released within the downcomer, and, in fact, the majority of said vapor must be released otherwise the downcomer can not accommodate the liquid/vapor mixture and will flood, thus preventing successful tower operation. This phenomena is generally deemed to occur when operating pressure is such as to produce a vapor density above about 1.0 lbs/cu. ft. and typically amounts to about 10 to 20% of the vapor by volume. For conventional trays, as shown below, the released vapor must oppose the descending frothy vapor/liquid mixture flowing over the weir into the downcomer. In many cases, such opposition leads to poor tower operation and premature flooding.
The technology of gas-liquid contact addresses many performance issues. Certain performance and design issues are seen in the publication "Ballast Tray Design Manual," Bulletin No. 4900-Fifth Edition by Glitsch, Inc., assignee of the present invention. Other examples are seen in several prior art patents, which include U.S. Pat. Nos. 3,959,419, 4,604,247 and 4,597,916, each assigned to the assignee of the present invention and U.S. Pat. No. 4,603,022 issued to Mitsubishi Jukogyo Kabushiki Kaisha of Tokyo, Japan. A particularly relevant reference is seen in U.S. Pat. No. 4,499,035 assigned to Union Carbide Corporation that teaches a gas-liquid contacting tray with improved inlet bubbling means. A cross-flow tray of the type described above is therein shown with improved means for initiating bubble activity at the tray inlet comprising spaced apart, imperforate wall members extending substantially vertically upwardly and transverse to the liquid flow path. The structural configuration is said to promote activity over a larger tray surface than that afforded by simple perforated tray assemblies. This is accomplished in part by providing a raised region adjacent the downcomer area for facilitating vapor ascension therethrough.
U.S. Pat. No. 4,550,000 assigned to Shell Oil Company teaches apparatus for contacting a liquid with a gas in a relationship between vertically stacked trays in a tower. The apertures in a given tray are provided for the passage of gas in a manner less hampered by liquid coming from a discharge means of the next upper tray. This is provided by perforated housings secured to the tray deck beneath the downcomers for breaking up the descending liquid flow. Such advances in tray designs improve efficiency within the confines of prior art structures. Likewise, U.S. Pat. No. 4,543,219 assigned to Nippon Kayaku Kabushiki Kaisha of Tokyo, Japan teaches a baffle tray tower. The operational parameters of high gas-liquid contact efficiency and the need for low pressure loss are set forth. Such references are useful in illustrating the need for high efficiency vapor liquid contact in tray process towers. U.S. Pat. No. 4,504,426 issued to Karl T. Chuang et. al. and assigned to Atomic Energy of Canada Limited is yet another example of gas-liquid contacting apparatus.
Several prior patents have specifically addressed the tray design and the apertures in the active tray deck area itself. For example, U.S. Pat. No. 3,146,280 is a 1964 patent teaching a directional float valve. The vapor is induced to discharge from the inclined valve in a predefined direction depending on the orientation of the valve in the tray deck. Such valve configurations are often designed for particular applications and flow characteristics. Tray valves with weighted sides and various shapes have thus found widespread acceptance in the prior art. A circular valve structure is shown in U.S. Pat. No. 3,287,004 while a rectangular valve structure is shown in U.S. Pat. No. 2,951,691. Both of these patents issuing to I. E. Nutter, teach specific aspects of vapor liquid contact flow utilizing tray valve systems. Such specialized designs are necessary because vapor-liquid flow problems must be considered for each application in which a tray is fed by a downcomer. The type of directional flow valve, its orientation, and its predisposition to vapor-liquid flow interaction are some of the issues addressed by the present invention.
It would be an advantage to provide a method of and apparatus for enhanced vapor liquid flow manifesting increased efficiency with a directional thrust valve assembly. Such a valve tray assembly is provided by the present invention wherein a circular tray valve is supported by first and second support legs, oriented into the liquid flow of the tray with the first leg having a wider surface area presented to the flow for diverting the flow therearound. The width of the first leg is substantially less than the diameter of the circular valve aperture, about which the liquid is induced to flow into engagement with the vapor passing therethrough. This valve assembly, when used in conjunction with, and outwardly of, a raised active inlet area further controls initially directed liquid flow from the active inlet area beneath the downcomer.